In a typical cellular radio system, also referred to as a wireless communication system, user equipments, also known as mobile terminals and/or wireless terminals communicate via a Radio Access Network (RAN) to one or more core networks. The user equipments may be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability, e.g., mobile termination, which user equipments may be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data via a radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is referred to as “eNB”, “NodeB” or “B node” and which in this document is referred to as a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. The base station communicates over the air interface operating on radio frequencies with the user equipments within range of the base station.
In some versions of the radio access network, several base stations are typically connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. In the end of 2008 the first release, Release 8, of the 3GPP Long Term Evolution (LTE) standard was finalized and Release 9 is currently being specified. Evolved Universal Terrestrial Radio Access (E-UTRA) is the air interface that is used in the LTE.
In a packet-based communications system using Automatic Repeat Request (ARQ), received packets are required to be acknowledged by the receiver, i.e. a message shall be sent from the receiver to the transmitter telling whether the packet was received correctly or not.
In e.g. US2006109810, preamble miss detection in a communication system using ARQ processes is depicted. The communication system transmits data packets from a sender to a receiver using hybrid automatic repeat request processes. The sender redundantly encodes each packet, divides the packet into sub packets, and sends the sub packets to the receiver in a time-interlaced manner. When the receiver returns a positive acknowledgement of a sub packet using an acknowledgement channel, the sender terminates transmission of the sub packets. The sender interprets the signals on the acknowledgement channel using a metric resulting from correlation of the signals with positive and negative acknowledgement symbols. The sender interprets low correlation of the acknowledgement channel signal with both positive and negative acknowledgement symbols as a preamble miss, and terminates transmission of the subpackets. After termination, the packet may be rescheduled for transmission. Early termination of packet transmission after a preamble miss improves bandwidth utilization and decreases latency of the packet with the missed preamble.
In the physical layer of E-UTRA a certain type of ARQ known as Hybrid ARQ (HARQ) is implemented both in uplink and downlink. Uplink (UL) is the portion of a communications link used for the transmission of signals from a user equipment to a base station. Downlink (DL) is the portion of a communications link used for the transmission of signals from a base station to a user equipment. The acknowledgement message in E-UTRA is denoted HARQ-ACK. In the following only HARQ-ACK transmitted in uplink is considered.
HARQ-ACK may be transmitted by the user equipment in response to certain Physical Downlink Shared CHannel (PDSCH) transmissions and includes one or several acknowledgements, either positive (ACK) or negative (NACK) in response to transport blocks transmitted in the downlink, which HARQ-ACK is indicated by a downlink grant. HARQ-ACK may be transmitted on one of the physical channels Physical Uplink Control CHannel (PUCCH) or Physical Uplink Shared CHannel (PUSCH). When HARQ-ACK is transmitted in a subframe in which the user equipment is scheduled for transmission, the HARQ-ACK may be multiplexed with data and/or other control information and transmitted on PUSCH to the base station.
Different modulation schemes and coding may be used for transmitting subframes of data on PUSCH depending on signal quality and cell usage. Quadrature Phase Shift Keying (QPSK) is commonly used, but in good radio conditions 16 Quadrature Amplitude Modulation (16 QAM) and 64 QAM may be used which increases data throughput rates, wherein QPSK has 4 constellation points; 16 QAM has 16 constellation points; 64 QAM has 64 constellation points.
For Frequency Domain Duplex (FDD) and for Time Domain Duplex (TDD) ACK/NACK bundling of the HARQ-ACK information comprises one or two bits. ACK/NACK bundling is achieved by acknowledging several transport blocks with one acknowledgement per predefined set of transport blocks. If all the transport blocks in the set are received correctly, the acknowledgement is positive (ACK), otherwise it is negative (NACK). The encoding of HARQ-ACK transmitted on PUSCH is described in the following. Each positive acknowledgement (ACK) is encoded as a binary ‘1’, and each negative acknowledgement (NACK) is encoded as a binary ‘0’. If HARQ-ACK comprises 1-bit of information, i.e., [o0ACK], it is first encoded according to Table 1 where Qm is the number of bits per symbol, i.e. Qm=2, 4, and 6 for QPSK, 16 QAM, and 64 QAM, respectively. If HARQ-ACK comprises 2-bits of information, i.e., [o0ACK o1ACK], it is first encoded according to Table 2 where o2ACK=(o0ACK+o1ACK)mod 2.
TABLE 1Encoding of 1-bit HARQ-ACKEncoded HARQ-QmACK2[O0ACK y]4[O0ACK y x x]6[O0ACK y x x x x]
TABLE 2Encoding of 2-bit HARQ-ACKQmEncoded HARQ-ACK2[O0ACK O1ACK O2ACK O0ACK O1ACK O2ACK]4[O0ACK O1ACK X X O2ACK O0ACK X X O1ACK O2ACK X X]6[O0ACK O1ACK x x x x O2ACK O0ACK x x x x O1ACK O2ACK x x x x]
The “x” and “y” in Table 1 and 2 are placeholders for scrambling the HARQ-ACK bits in correlation with 3GPP TS 36.211, in a way that maximizes the Euclidean distance of the modulation symbols carrying HARQ-ACK information. In particular “x” will be transmitted as ‘1’ after scrambling, i.e. the last Qm−2 bits mapped on a modulation symbol are all ‘1’s. As a result the HARQ-ACK is mapped only on the four corners of the constellation. The constellation for 16 QAM is shown in FIG. 1 where the mapping of the bits for HARQ-ACK is shown with filled circles.
The HARQ-ACK information is transmitted in QACK bits in Q′ symbols, where QACK=Q′Qm, and the codeword qjACK, j=0,1, . . . , QACK−1 is obtained by concatenation of multiple encoded HARQ-ACK. QACK is the number of bits and Q′ is the number of symbols.
The PUSCH transmission with HARQ-ACK differs from the PUSCH transmission without HARQ-ACK only in that in some modulation symbols the data or other control information is punctured and replaced by HARQ-ACK.
When the base station anticipates HARQ-ACK on PUSCH, the modulation symbols are demultiplexed and the HARQ-ACK modulation symbols are used by the HARQ-ACK detector. However, it might be that the user equipment transmits data and/or other control information on PUSCH in response to an uplink grant, but that the UE has not received the downlink grant for receiving a transmission from the base station. As a consequence the UE will not transmit HARQ-ACK but data or other control information in the modulation symbols intended for HARQ-ACK. To send nothing at all, data or other control information, in the modulation symbols intended for HARQ-ACK behaviour, is denoted DTX.
If the base station detects ACK instead of Discontinuous Transmission (DTX), so called ACK false detection, the base station will erroneously consider the corresponding downlink transport block as correctly received. Since the transport block has not been correctly received by the UE corresponding data will not be passed to the Medium Access Control (MAC) layer and from the MAC layer to the Radio Link Control (RLC) layer. Data will hence be missing in the RLC layer. This will cause ARQ retransmissions in the RLC layer which introduce delay and possibly large retransmissions and hence is undesirable. Also, if a NACK is detected erroneous that in reality is DTX, the base station will retransmit the packet in such a way that the user equipment will not be capable to decode it. It is thus a problem for the HARQ-ACK detector for PUSCH in the eNodeB, if it can not distinguish between data and HARQ-ACK and decode the correct HARQ-ACK message if present.